1. Field of the Invention
The present invention relates to an optical pickup device and a method of laying out an optical system thereof, which is in particular suitable for use in a compatible optical pickup device capable of outputting laser light of a plurality of wavelengths as well as a method of laying out such a device.
2. Description of Related Art
With the growing size of optical disks in their capacity, development of HD-DVDs (High Definition Digital Versatile Discs) using blue wavelength laser light has been pursued. Presently, optical pickup devices compatible for both of such HD-DVDs and existing DVDs have been under development.
A typical configuration for such compatible optical pickup devices of this kind, as described in International Patent Publication No. WO97/42631, for example, may be such that separate objective lenses are positioned respectively for HD-DVD laser light and DVD laser light. In this configuration, the two objective lenses may be disposed side by side along a direction orthogonal to a direction of the disk diameter. In such a case, when one of the objective lenses moves along the disk diameter, the other objective lens moves along a line in parallel to and with a specific distance from the disk diameter. However, in this manner, the relation between the other objective lens and a track direction of the disk varies in accordance with the movement of the objective lenses, as described below.
FIGS. 16A and 16B show angles of the track direction at respective positions as the two objective lenses moves along the disk diameter.
In FIG. 16A, one of the two objective lenses moves along a line O-Y (amount of normal misalignment=0), while the other objective lens moves in parallel to the line O-Y with a distance z from the line O-Y (amount of normal misalignment=Z). At this time, supposing that an angle of the track direction is θ2 (an angle formed by the meeting of a tracking tangent with a line parallel to the OX line) in FIG. 16A, a relation between an amount of displacement in the objective lenses and a track angle is as shown in FIG. 16B. Note that, FIG. 16B shows an example when the amount of normal misalignment is 5 mm.
As shown in the drawings, if no normal misalignment occurs in the objective lenses, the track angle θ2 is always equal to zero regardless of the positions of the objective lenses. In contrast, if the normal misalignment occurs in the objective lenses, the track angle θ2 changes as the objective lenses move as also shown in the drawing. This change in the track angle θ2 also changes a direction of the track on a photodetector. This can cause such a problem that the track direction rotates with respect to a parting line of a sensor pattern, hindering generation of a proper push-pull signal.
Note that, in FIG. 16A, the track angle θ2 satisfies the relation of θ2=θ1. Accordingly, the track angle may also be represented as θ1.
The optical pickup devices of this type, as shown in FIG. 17, may adopt a configuration in which a half mirror in a plate shape is inserted into an optical system for which an oscillation angle α is provided, thereby introducing astigmatism to the laser light as a result of refracting effect of the half mirror. With this configuration, it is possible to make an optical system compact in size, and to reduce the number of components.
However, in this case, there is often a case in which the oscillation angle α may not be set to ±45° because of a laying out condition of optical components. In this case, the track direction on the photodetector does not align with a 45° direction with respect to a deformation direction of a beam spot due to the astigmatism. Consequently, a problem arises that a 4-split sensor may not be positioned at a position at which both a focus error signal and a tracking error signal (push-pull signal) become appropriate.
Specifically, if the 4-split sensor is positioned so that the parting line becomes 45° with respect to the deformation direction of the beam spot, the track direction on the sensor is inclined with respect to the parting line of the sensor pattern. In contrast, if the 4-split sensor is positioned so that the parting line of the sensor pattern is along the track direction, this parting line does not align with a 45° direction with respect to the deformation direction of the beam spot.
Moreover, as described above, if the normal misalignment occurs in the objective lenses, the relation between the deformation direction of the beam spot due to the astigmatism and the track direction may further grow worse, depending on the direction and the degree of the normal misalignment. In such a case, adjusting the parting line of the sensor pattern to be appropriate for detection of the astigmatism causes such problems that a proper push-pull signal may not be generated, and a smooth tracking control may not be performed.